Nutritional evaluation of dehulling and thermal treatments for production of expeller soybean meal

The objective of this study was to determine the influence of different processes on the nutritional quality of expeller soybean meal (SBM). Extrusion or cooking processes in combination with dehulling and pressing were used to produce 4 partly defatted SBM with low residual trypsin inhibitor (TI) activity. European soybeans were ground using a roller mill, then dehulled or not with a cleaner separator. The 2 products were either extruded using single-screw extrusion at 140°C at around 100 kg/h (EP), or cooked at 150°C during 60 minutes (FCP) after flaking. All beans were then pressed to extract the oil. Extrusion allowed a higher oil extraction than cooking (residual oil: 4.9 vs 8.6 g/100 g DM in whole EP and FCP meals, and 5.2 vs 6.4 g/100 g DM in dehulled EP and FCP meals, respectively). The dehulling step resulted in an increase of almost 3 g/100 g protein (58.8 and 58.4 g/100 g for dehulled EP and FCP meals, respectively, and 56.0 and 55.8 g/100 g for whole EP and FCP meals, respectively, on a fat free DM basis). The KOH protein solubility was increased by dehulling for EP (70 vs 76%) and FCP processes (82 vs 89%, for whole and dehulled meals, respectively). The TI values were 2.6, 3.5, 3.6, and 7.6 TIU/mg for whole and dehulled EP and FCP meals, respectively. The lower dryer outlet temperature measured for the dehulled beans compared to whole beans (90 vs. 97°C) may explain the lower TI inactivation for the dehulled FCP meal. The amino acid and reactive lysine content and the in vitro rate of degradation based on hydrolysis of protein bonds (pH-stat) are determined and related to the chemical composition to evaluate the nutritional value of the meals. All 4 processes may produce good quality SBM with 46-52 g/100 g crude protein, 4-8 g/100 g residual oil, and a TI content below 8 TIU/mg. Extruded-expelled SBM produced in medium-sized crushing plants from local and GMO-free crops have a nutritional and economic potential in Europe. This study is part of the Feed-a-Gene project and received funding from the European Union's H2020 program under grant agreement n° 633531.